Spotting during early pregnancy can be very normal. Sometimes it is accompanied by low back pain and cramping in the lower abdomen.
The endometrium builds up a thick bed of blood vessels and as the "blastocyst" implants, some blood is released from the uterine wall. Of course, this is typically how miscarriage begins as well, so it is a good idea to notify your care provider and monitor the spotting. For example, does it increase after exercise, sex, a busy day? If it does, it is best to limit such activities until the spotting goes away.
Bleeding in the second and third trimesters is much more ominous, but first trimester bleeding is very common. Some authors say it happens frequently when the next menses would have been due.